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Tomato Viruses: Leaf Blight. Symptoms and Solutions

Recognizing and Controlling Leaf Bronzing Disease in Tomato Plants for Optimal Organic Garden Health. Learn effective prevention and natural remedies.

by BioGrow

In this article, we want to delve into the tomato virosis known as leaf bronzing (Tomato Spotted Wilt Virus – TSWV). It is a tomato disease that is very difficult to recognize and eradicate. This is a significant problem during the summer when tomato crops reach their maturity. To control this disease, it is necessary to promptly recognize it and acquire knowledge of a series of techniques that will prevent its recurrence in the following years.

The tomato leaf bronzing virosis is not an easy disease to defeat. However, with proper precautions, this condition can be controlled.

What are viroses

Among tomato diseases, viroses are certainly the most formidable, for several reasons.
Firstly, they are not easily recognizable, except by those who have faced them at great cost in their own crops.
Secondly, it is not easy to understand the origin of the disease.
Thirdly, they are difficult and sometimes impossible to combat, both in organic and traditional agriculture, as they trigger an irreversible process in the crops that cannot be counteracted.
To defend against them, a thorough understanding of the problem is necessary. Additionally, a series of biological techniques must be implemented to prevent the recurrence of the tomato disease in future years.
The various tomato viroses share a particular common characteristic: they are obligate parasites. This means that they cannot live independently outside host plants.
To be transported from one plant to another, viroses need a vector, which is often a parasitic insect that spreads the disease. This is where we can intervene to contain the problem and prevent its reappearance in future organic crops.
Now, let’s look in detail at the main and most widespread tomato virosis.

Tomato Bronzing (Tomato Spotted Wilt Virus – TSWV)

Tomato virosis, known as tomato bronzing or spotted wilt of tomatoes, is certainly one of the most dangerous and difficult tomato diseases.

How tomato bronzing appears on the plant

Tomato plants affected by this virosis initially show spots on the apical part of the plant. Leaves and tops turn a bronze-like necrotic yellow, hence the name tomato bronzing virosis.

Leaf bronzing on young tomato plant

A young plant affected by the tomato virus known as “Leaf Bronzing”

The virus appears suddenly, sometimes in a disorganized manner, from one point to another in the cultivation.
As seen in the photo, the young plant wilts, and the leaves begin to necrotize. The tomato virosis can strike at any stage of the cultivar’s growth. If the plants are in the initial stage of growth, before the formation of flower clusters, the chances of resisting the disease are close to zero. The virus immediately stops their growth. The rapid necrosis of the vegetative apices does not allow the plant to recover.

Tomato viroses - young plants attacked by the virus

Young plant attacked by leaf bronzing

The root system of the plant is also affected by the virosis. As seen in the photos below, the roots are necrotic and dark-colored. The trunk is also infected, presenting abnormal brown streaks along the plant’s lymphatic veins.

Tomato viruses - necrosis in the root system of the plant

Necrosis in the root system of the plant

If the infection occurs at a later stage of development, growth proceeds in a stunted manner. The vegetative apices initially lose vigor and then transform into bronzing and leaf necrosis.
On immature fruits, the tomato disease presents itself with circular or concentric ring-shaped spots, raised and merging with each other. Mature tomatoes, on the other hand, appear deformed with yellow areas and are clearly no longer suitable for consumption.

How to deal with tomato bronzing attack

The “tomato bronzing” – TSWV virosis represents a serious problem for the crops affected by it. There are no effective treatments to cure the disease, not even with heavy chemical pesticides. Once the tomato disease is circulating, the only option is to carefully remove the affected plants. This way, the possibility of infecting other crops can be reduced. The virosis, in fact, affects not only tomato plants but also other important crops. These include lettuce, basil, and especially peppers.
Also, be cautious with your gardening tools. Do not use the same tools for healthy and infected plants, as it may accelerate the spread of the virus.

The vector of tomato virosis: Western Flower Thrips (Frankiniella occidentalis)

As we mentioned, the Tomato Spotted Wilt Virus virosis spreads rapidly through a vector on the cultivar. In this case, the vector of the disease is the feared Western Flower Thrips, Frankiniella Occidentalis.
As we saw with the Tomato Leaf Miner, the insect is native to the American continent. It appeared in European ecosystems only in the late 1980s.
The Western Flower Thrips is a polyphagous insect belonging to the Thrips family. Its extreme danger is due to several factors.
Firstly, its size; it is extremely small. The adult insect measures about 1 mm in length, making it very difficult to detect.

Western Flower Thrips Frankliniella occidentalis

Western Flower Thrips Frankliniella occidentalis

The active adults in the spring and summer are ochre in color, with darker streaks and punctures in the dorsal region. Overwintering forms are brownish, while young stages are very clear.
Another factor of danger is its reproduction speed: the Western Flower Thrips can lay from 20 to 100 eggs per day, from which nymphs emerge. The activity of the insect continues until the end of summer and sometimes extends into autumn. With our climate, the number of generations can reach up to 6 or 7 in a year.[/caption]

The damage caused to tomatoes by the virosis

In addition to being a vector of viroses, the tiny thrips that cunningly hides in the plant’s flowers causes direct damage, both from the bites of the insect and from the oviposition activity.
The presence of toxic substances in the insect’s saliva causes depigmentation, i.e., color loss, on the leaves. They initially become silvery and then necrotize.
The damage from oviposition is manifested by suberifications and deformations of the affected tissues.

damage by Western Flower Thrips on tomato leaf

Damage by Western Flower Thrips on tomato leaf

How to defend against tomato virosis transmitted by the Western Flower Thrips in organic farming

Defending against this dangerous tomato disease is not easy. To do so in organic farming, an integrated defense strategy must be implemented. This strategy starts with correct cultivation practices.

Mulching

When discussing organic tomato cultivation, we emphasize the importance of mulching and weed control.
Harmful weeds are an ideal environment for the development and proliferation of the Western Flower Thrips, the vector of the virosis. Absolute control of the weeds, therefore, limits the development of the phytophagous insect.

Chromotropic traps

Another biological defense technique to control and limit the proliferation of the thrips is to position chromotropic traps. The color that attracts the small and dreaded insect is blue. This strategy (100% organic, of course) is crucial for the defense of your plants. An affordable and functional purchase can be made directly from this link.
The traps should be placed with a certain intensity, 10-12 for every 100 square meters of field.

Macerates against tomato virosis

Another prevention action is the use of various macerates that we have learned about in our various articles. Namely, the
nettle macerate, the garlic infusion and the tomato macerate, to make the crops at risk less attractive to the insect.
It is advisable to continuously alternate the use of different macerates because the Western Flower Thrips has demonstrated strong resistance and adaptability even to chemical pesticides. Using the same type of natural preparation continuously could be counterproductive.

Action against the virus

What has been indicated serves for combating the vector of the tomato virosis. However, concerning the virus itself, it is necessary to implement a series of countermeasures at the end of cultivation. This is to prevent the virosis from occurring again in subsequent seasons.
As mentioned, it is essential to eliminate and destroy infected plants, and the best method is burning. Always, of course, where allowed by the modifications made to legislative decree 152 of April 3, 2006.
The recommendation for burning, of course, is always to apply all necessary procedures.

Soil disinfection

After cleaning, it may be useful to disinfect the soil using hydrated lime or sulfur. The substance should be applied directly to the affected crop surfaces of the tomato disease.
Since the virosis originates in the soil, it is advisable to leave the affected portion of our vegetable garden fallow at the end of cultivation. Also, carry out deep soil tillage in the winter. This way, you will kill the larval stages of the Western Flower Thrips and sterilize the soil through the action of frost.
Virosis generally appears on tired and organic matter-depleted soil. Therefore, after careful cultivation, make extensive use of animal manure. This way, you will regenerate the soil and start a new crop without the risk of encountering the virosis again.
We conclude this article by hoping that your vegetable garden will never be affected by this dreaded disease. We look forward to subsequent articles for the study and understanding of other tomato diseases.

Further Reading

  • Tomato Viruses from University of Minnesota Extension: This resource discusses more than a dozen viruses that can infect tomatoes. The most common viruses in Minnesota are tomato mosaic virus (ToMV) and tobacco mosaic virus (TMV).
  • Tomato Spotted Wilt / Tomato / Agriculture – UC IPM from University of California Agriculture and Natural Resources: This resource provides information about Tomato spotted wilt virus, which causes bronzing of the upper sides of young leaves, which later develop distinct, necrotic spots.
  • Tomato brown rugose fruit virus (ToBRFV): A new concern for … from Michigan State University Extension: This article discusses Tomato brown rugose fruit virus (ToBRFV), a new concern for tomato and pepper producers. Currently, no commercial tomato varieties are tolerant to ToBRFV.

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